The present invention relates generally to methods for detecting the presence of hidden electronic devices, and more particularly relates to methods for detecting the presence of a radio-frequency device having a frequency response including a band-edge characteristic, using reflected nonlinear reflection signals.
In the past, remote detection of radio-frequency (RF) devices has focused on passive sensing of signals radiated by the target device, with a few simple active detection techniques used as well. A typical passive method employs direction finding techniques to determine the location of a transmitting RF device, using multiple signal receivers or multiple antennas connected to a single receiver. This method, of course, will fail if the hidden electronic device does not transmit RF signals, or is switched off. Active techniques, on the other hand, typically rely on nonlinear junction detection, and may be used even when the target device does not transmit or leak any RF signals, or when the device is switched off. For instance, U.S. Pat. No. 6,057,765, issued to Jones et al. on May 2, 2000, describes a nonlinear junction detector that transmits an RF signal and measures signal harmonics re-radiated by semiconductor devices included in the target device. However, this scheme suffers from false detections that frequently arise from the presence of non-electronic nonlinear junctions in the target area. For example, in an indoor environment, oxidized fasteners commonly used in constructing interior walls may cause false alarms.